1. Field of the Invention
This invention generally relates to a fastener driving tool and more particularly to a pneumatically operated fastener driving tool having a pivotally mounted magazine interlocked with a trigger assembly which includes a cam lever for amplifying the displacement of the magazine assembly and a snap action poppet valve assembly for controlling the compressed air supply to the drive piston.
2. Description of the Prior Art
Fastener driving tools are generally known in the art. Some such tools include a trigger interlock which prevents operation of the tool unless it is in engagement with a workpiece. More specifically, in some known tools a safety yoke is provided which extends downwardly from the nosepiece. Such safety yokes generally include an integrally formed lever which actuates a trigger pin when the nosepiece is in engagement with the workpiece. Such tools cannot be operated unless the trigger pin is actuated.
In other known tools a pivotally mounted magazine is provided instead of a safety yoke. Such tools are generally used in applications where a safety yoke would be awkward and cumbersome. An example of such a tool with a pivotally mounted magazine is disclosed in U.S. Pat. No. 3,638,532, assigned to the same assignee as the present invention and hereby incorporated by reference. In such tools, the displacement of the magazine is relatively small. Since this displacement is necessary to actuate the trigger valve, it is necessary to maintain a relatively close tolerance of the components which comprise the interlock to prevent improper trigger valve timing. More specifically, the trigger valve controls the driving of a fastener into a workpiece. If the operation is premature (i.e., the tool is operated before the magazine is in the operate position), this can result in inadequate follow-through of the driver blade causing the fastener to be improperly driven into a workpiece. On the other hand, if the valve timing is delayed, the driver blade follow-through could result in an undesirable multiple operation condition. Accordingly, in order to solve such problems, known fastener driving tools utilize relatively close tolerance components used for the trigger interlocks. However, such components can be relatively expensive resulting in a relatively higher cost tool.
Another problem with pneumatically operated fastener driving tools is known as poppet flutter. Poppet valves are used to control the compressed air flow into a drive cylinder which houses the drive piston which has a driver blade rigidly attached thereto. The type of poppet valve in question has what is known as a fixed differential. More specifically, the area on which pressure acts remains constant regardless if the poppet is open or closed. In the static position, the poppet valve is closed and consequently the air passageway to the drive cylinder is sealed off. In this position, a spring and compressed air bias the poppet valve closed. In a drive position an exhaust passageway is opened to release the air bias on the poppet. This uncontrolled release of the air bias can result in fluttering of the poppet valve possibly causing a misoperation of the tool. More specifically, when the air bias is not controlled when released, a constant differential pressure is created across the poppet valve which can cause the poppet valve to flutter if the pressure release is not controlled. Additionally, the constant differential poppet eliminates the use of a relatively larger varying differential poppet which would be needed for the required air flow. The use of a relatively larger poppet results in a dimensionally larger and more expensive tool.